The present invention relates, in general, to a method and apparatus for injecting electrical current into the earth surrounding a target well and for detecting and measuring magnetic fields produced by such currents during drilling of a relief well. More particularly, the invention relates to a system for injecting alternating currents into the earth and for measuring magnetic fields produced by current flow in nearby cased wells, to determine the distance and direction to such wells, such measurements being accomplished from within a drill string so that removal of the drill string from the well for logging purposes is not required.
In deep well drilling, where wells are commonly drilled to depths in excess of 12,000 ft., it is extremely difficult to track with accuracy the path followed by the drill, since even a very minor deviation in direction can, after several thousand feet of drilling, produce a huge divergence between the actual and intended locations. Because of variations in geologic formations, the high temperatures encountered, and the generally unfavorable conditions which exist in the environment of a deep well, accurate tracking of a well during the course of drilling is extremely difficult. Although numerous techniques have been developed for making such measurements, none have been found to be entirely satisfactory.
Although an accurate determination of well location is always desirable, such information becomes particularly critical in two related situations: first, when an attempt is being made to locate and intercept a well, and second, when an attempt is being made to avoid another well which is known to be located in the general area being drilled. The first situation may occur, for example, when an existing well blows out and it becomes necessary to drill a relief borehole that will intersect the existing, or target well. In these cases, the relief borehole must be started a long distance from the target wellhead; for example, as much as one-half mile away, and must angle down to intersect the target well at a depth of, for example, 10,000 to 15,000 feet. Although the initial part of the drilling can be done using existing directional equipment, a problem arises when the relief borehole nears the target well. Not only must the measurements be made at locations which are thousands of feet below the surface of the earth, but in addition the exact relative locations of the borehole and the well often are not known with sufficient accuracy. The actual paths of both the target well and the relief borehole can deviate substantially from their desired paths during the drilling process, and such deviations often are not measured accurately, particularly during the drilling of the target well. As a result, the target well might not be in its expected location, and the relief borehole can easily intersect it prematurely, or can miss it entirely. Furthermore, even if the location of the target well is accurately known, a slight deviation from the assumed location and direction of the relief well can result in premature intersection with the target well or can cause the relief well to miss the target well entirely.
The second situation noted above, i.e., the need to avoid existing wells, occurs, for example, when several wells are being drilled from a single location, as from an off-shore platform. The problem usually arises when it is desired to add a well between the existing wells while avoiding contact with them. The initial drilling from an off-shore drilling platform may locate a number of well heads at 12 foot intervals, with the wells themselves being directed away from each other in various directions. In order to increase the production from such a drilling platform, it may become desirable to drill new wells between the existing wells so that the well heads will be at 6 foot intervals. However, it is essential that the new wells avoid the existing wells, but even if great care is taken to monitor the location of the drill and the direction of drilling, there is often uncertainty about the exact location of the drill with respect to the existing wells.
It is known that the magnetic and electrical characteristics of the geological formations surrounding a well being drilled can be measured by means of highly sensitive magnetometer systems. In such systems, for example, electrical currents are caused to flow in the strata surrounding the well, with the current flow creating a magnetic field that can be measured. The injected currents do not flow uniformly through the strata, but may be concentrated in conductive anomalies such as the metal casing of a target well, thereby creating a detectable variation in the magnetic field. Such a system is disclosed in U.S. Pat. No. 4,372,398, issued February 1983 and entitled "Method of Determining the Location of a Deep-Well Casing by Magnetic Field Sensing". This patent describes a method for injecting current into the earth surrounding the relief well, with the current flowing to and along the target. The magnetic field resulting from the target current is measured by a highly sensitive magnetic field sensor such as that shown in U.S. Pat. No. 4,323,848, issued Apr. 6, 1982, and entitled "Plural Sensor Magnetometer Arrangement for Extended Lateral Range Electrical Conductivity Logging".
Although such prior systems have been effective, a difficulty has been encountered because of the fact that they cannot provide measurements during the actual drilling of the relief well since they require removal of the drill string and insertion of the measuring equipment to allow logging measurements to be made. Because of the extreme depth of such wells, the removal of a drill string is extremely expensive, and results in a large amount of lost drilling time. Such lost time is particularly critical when an attempt is being made to intercept and shut down a blown out well. Furthermore, on occasion the logging equipment itself becomes stuck in the relief well, further delaying drilling and increasing costs while attempts are made to free it. Since logging measurements must be made more and more often as the relief well approaches the target well, the costs and time delays involved become considerable.
In U.S. Pat. No. 4,529,939, issued Jul. 16, 1985 to Arthur F. Kuckes, and entitled "System Located in Drill String for Well Logging While Drilling", the disclosure of which is hereby incorporated herein by reference, there is described an MWD (measurement while drilling) system for injecting alternating current into strata surrounding a borehole being drilled to produce a current flow in the casing of a target well. This target well current results in a measurable target magnetic field. The system further includes a magnetic field sensor tool within the drill string of the borehole being drilled for measuring the field to determine the distance and direction to the target well casing, the measurement being made without withdrawing the drill string from the borehole being drilled. The bottom-most section of the drill string carries a conventional dynamotor drill bit which may be incorporated in a bent subsection for directional drilling, or in other conventional drill string subsections. Drilling mud under high pressure is supplied through the drill string and through a conventional turbine arrangement in the bent subsection to drive the drill bit. In conventional manner, the mud then flows out of the drill string and back up the borehole being drilled, outside the wall of the drill string, to return to the surface. The drill string is assembled and lowered into the well by means of a suitable derrick, or the like, located at the wellhead, in known manner.
The well logging apparatus of the '939 patent is suspended within the central passageway of the drill string and is supported near the bottom of the drill string, just above the subsection which carries the drill bit. The logging apparatus may be held in place in the drill string by means of a suitable support cable extending the full length of the drill string, or may be a self-contained unit secured in the bottom section of the drill string. In either event, the logging apparatus includes an alternating current emitting electrode located in the drill string bore and a magnetic field sensor tool located a predetermined distance below the electrode.
In a conventional drill string, each drill pipe subsection, or collar, is approximately 30 feet in length, with three subsections making up a full section. In accordance with conventional drilling techniques, the lowermost sections, making up about 300 feet of pipe, may weigh from 100 to 150 pounds per foot. An intermediate portion of the drill string, perhaps the next 600 feet, is conventionally constructed of pipe in the range of 28 pounds per foot, with the remainder of the drill string being constructed of drill pipe having a weight of 15 pounds per foot. The heavy weight of the lower pipe sections is not only for strength, but also serves to place a downward force on the drill bit. At the very bottom of the drill string is a modified collar, or sub, which carries the drill bit. The bit may either be vertical, for straight drilling, or may be angled with respect to the axis of the drill string for directional drilling (i.e., in a "bent" sub), the string being rotated from the surface to orient the drill so that it will advance in the desired direction.
In accordance with the '939 patent, the drill string collar immediately above the drill bit sub is a sensor collar constructed of a non-magnetic high resistivity material such as stainless steel or Monel. This sensor collar is adapted to receive a highly sensitive magnetic field sensor tool which is oriented within the sensor collar by means of a detent or the like. This detent assures a fixed angular relationship between the sensor and the drill string so that when a bent sub is utilized, its direction of travel can be accurately determined by the sensor.
In order to minimize the effect of magnetic fields produced by current flow in the sensor collar itself, the magnetic field sensing tool is held in coaxial relationship with the drill string by means of suitable spacers. Furthermore, the sensor collar is carefully constructed to insure that its inner and outer surfaces are concentric so that any current flow that does occur is equalized around the circumference of the collar, thereby insuring that any stray magnetic fields created in the relief borehole by such a current are essentially cancelled at the axis of the drill string, so that they have minimal or no effect on the field sensor tool.
Current is injected into the earth surrounding the relief borehole, in accordance with the '939 patent, by means of an electrode located within the central bore of the drill string and spaced above the field sensor by a distance sufficient to insure that the current flow within the drill string will not adversely affect the desired field readings. The electrode is preferably mounted within the drill string approximately 70 to 150 feet above the field sensor tool. The electrode might be as close as 30 feet from the field sensor in some cases; generally speaking, however, the further away the electrode is from the sensor, the better. Since the electrical resistance of the drill pipe sections is sufficiently high to force all of the current emitted by the electrode into the surrounding earth within a distance of about 200 feet from the electrode, placing the electrode as much as 300 feet away from the sensor insures that essentially no current remains in the drill string itself in the area of the field sensor, and such an arrangement would be preferred in circumstances where the stray magnetic fields at the sensor tool are to be minimized. However, when operating within, for example, about 10 feet of a cased target well, the magnetic field resulting from current flow in the target is sufficiently strong that the electrode can be quite close to the field sensor without adverse effect from the stray fields produced by current in the drill string. The controlling factor is the magnitude of the magnetic field to be detected.
The provision of a current-injecting electrode in a drill string, although presenting many advantages when the wellhead of a target well is inaccessible, has some disadvantages, since there are current losses in the earth surrounding the relief well which result in a reduced current flow at the target well and which, therefore, produce a smaller target magnetic field for detection by the magnetometer carried by the drill. Furthermore, when the current source is located in the relief well, current flow in the earth around the relief well produces a magnetic field which can interfere with the field produced by current in the target well casing. The smaller the relative magnitude of the target field, the more difficult it is to track, and thus the more difficult it becomes to guide the relief well.
When the wellhead of the target well is accessible, the current injecting electrode can be removed from the drill string and placed on the surface of the earth, near or in contact with the wellhead. A second, ground electrode is spaced as much as one-half mile away from the wellhead.
Preferably, the injecting electrode is connected to the casing of the target well or is placed as close as possible to the wellhead. This injecting electrode is connected by a cable through a source of low frequency alternating current to the ground electrode spaced far away from the target wellhead. The electrical current from the AC source is thus supplied to the casing of the target well and flows downwardly along the casing. By placing the ground electrode at a relatively large distance from the current injecting electrode, the return ground current flow from the target casing is spread out over the length of the casing.
The concentrated current flow in the target well casing produces a corresponding alternating magnetic field in the earth surrounding the casing. If the target well is vertical, the magnetic field is essentially horizontal, and is detectable by a highly sensitive magnetometer located in the borehole being drilled. Although the current flowing in the target well "bleeds" into the surrounding earth, the location of the injecting electrode at or adjacent the wellhead produces sufficient current flow at the depths of interest to provide a magnetic field that can be accurately and reliably detected in the relief borehole by such a magnetometer.
One of the major problems encountered in measurement while drilling (MWD) systems is the transmission of data from the magnetic field sensor in the drill string to computing equipment on the surface. Attempts have been made to transmit data bits representing the detected magnetic field to the surface by means of a transmission line or cable extending the length of the drill string, in the manner generally described in the '939 patent discussed above. However, the environment within a drill string is hostile to such electrical cables and to the signals which they carry, with the result that data can be lost, or the received data can be incorrect. This can produce significant errors in the calculations of distance and direction, and can result in erroneous instructions to the drill operator, producing at best a loss in time and money and at worst a premature intersection with a blow-out well and consequent injury to the drilling crew.
In order to avoid the problems of direct transmission of data on a wireline, attempts have been made to transmit the data by way of pressure pulses in the drilling mud, in the manner described in U.S. Pat. No. 4,021,774 to Asmundson et al. However, the amount of information that can be transmitted by this technology is very limited, since only about one to about ten pulses per second are available, and such a bit rate is completely inadequate for the amount of data required to produce accurate and reliable distance and direction calculations in a measurement-while-drilling system. The reason for this is that the magnetic field being detected at the drill string is very small, on the order of 100 microamperes per meter, and to measure such fields it is necessary to accumulate readings over a relatively long period of time, and to average the signals over that time. This is usually done in a computer located at the surface, but with a bit rate of one to ten pulses a second, such averaging is impractical.
One suggested solution to the foregoing difficulties is the use of downhole processing, where data averaging is carried out before the signals are transmitted uphole. However, difficulties occur even in downhole processing due to the fact that it is extremely difficult to maintain synchronization between the source which produces the ground currents and the downhole processing circuitry. Without proper synchronization, the phase of the magnetic field signals being detected is not known, and there is a 180.degree. ambiguity in the resulting data. Fluid telemetry cannot be used to provide the necessary synchronization because its data rate is too low, as noted above. Synchronized clocks at the surface and downhole are not generally a satisfactory solution, because even quartz-controlled clocks drift, particularly in the conditions which exist at the bottom of a borehole. Even with one part per million accuracy, the drift of a quartz-controlled clock is 1/10 second per day.